The entire description here below is situated in the context of active goggles for viewing of digital contents in relief (3D viewing).
Such a goggle set classically has a shutter based on at least one liquid-crystal-based cell.
More specifically, the shutters fitted into goggles of this type consist of at least one cell formed by two substrates placed so as to be facing each other and enclosing a liquid crystal. A majority of such substrates are made of a rigid and transparent material such as glass.
To make a shutter that is both light and compact, it is the common practice to use liquid-crystal-based cells which are light and compact. To reduce the weight of this type of cell, the classic practice is to reduce the thickness of the glass substrates by using glass substrates of small thickness, better known as thin glass substrates whose thickness classically ranges from 0.5 to 1 mm.
Such a goggle set are described especially in the patent application U.S. Pat. No. 6,943,852. This goggle set consists of a single cell formed by a first and second substrate situated so as to be facing each other. The space between these two substrates forms a cavity receiving a nematic type of liquid crystal.
It may be recalled that liquid-crystal-based materials are materials whose optical properties, especially their birefringence, can be modified by the application to these materials of an electrical field E created by the application of a control voltage V between the two substrates using transparent thin electrodes. When the liquid crystal is placed for example between a polarizer and an analyzer that are crossed, then a voltage-controlled optical shutter is obtained. A liquid-crystal-based optical shutter of this kind then has at least two states depending on whether or not a voltage is applied to the terminals of the two substrates:                a state known as a “passing” state in which it allows light to pass through, and        a “blocking” state in which it does not allow light to pass through or allows a very small part of this light to pass through.        
The requirement of quality in the rendering of motion in 3D vision has led for example to the development of techniques of triple-flash projection timed at 144 Hz instead of a double-flash technology timed at 96 Hz (i.e. an image projected alternately on each eye at a rate that cannot be perceived by the eye) for a standard sampling cycle of the cinema at 48 Hz.
Such techniques therefore dictate a technology of fast ocular shutters and an optical quality comparable with cinema hall projection quality dictated by Hollywood studios.
However, the classically used nematic liquid crystals cells do not make it possible to obtain a sufficiently short response time for the passage from the blocking state to the passing state known as the transparent state (also called aperture time). Indeed, this technology is limited to aperture times of the order of a few milliseconds. The nematic technology is in this case compatible only with the double-flash projection system timed at 96 Hz and not with triple-flash systems timed at 144 Hz.
Another technology used to make dynamic shutters is based on the use of smectic liquid crystals. This smectic technology makes it possible to obtain:                a short response time ranging from about 100 ms to a few hundred μs for the passage from the transparent state to the blocking state;        a short response time ranging from about 100 ms to a few hundred μs for the passage from the blocking state to the transparent state.        
The symmetry of switching between the blocking state and the transparent state will be noted. For example, ferroelectric smectic liquid crystals (FLC) or else anti-ferroelectric smectic liquid crystals (AFLC) are thus faster than nematic liquid crystals.
The use of faster liquid crystals in the passage from nematic technology to smectic technology as well as the increase in the beat frequencies (passing from 48 images per second to 144 images per second) entails higher control voltages for the liquid crystal for shorter rise times. It must be known that the liquid crystal thicknesses are also smaller: 1-2 μm for FLCs as compared with 5-10 μm for the different nematics. The result thereof is that the fields applied are higher (10V/μm for smectic liquid crystals to a few V/μm for nematic liquid crystals).
Thus, the increase in the beat frequencies and the electrical fields applied to the liquid crystal are accompanied by a noise source that proves to be inconvenient for the user. Indeed, given the proximity of the viewing goggle set (in this particular example) and of the use's auditory apparatus, the switching noises of the liquid crystal cells become troublesome when the liquid crystal cells are in proximity to the hearing equipment.
More specifically, whether it is in nematic technology or smectic technology, noise is caused chiefly by the vibration of the glass substrates when, in order to make the liquid crystal switch over, alternately positive and negative electrical voltages (in square-wave form) are applied to the glass substrates constituting a shutter cell (in the case of a viewing goggle) and to the glass substrates of the display unit (for example a portable telephone screen).
Indeed, under a control voltage V, the resulting electrostatic force exerted on the glass substrates at a frequency situated in the audio range causes the substrates to vibrate, the result of which is an acoustic vibration in the audible spectrum.
It can be shown that the amplitude of the vibrations induced by this electrostatic force is all the greater as the glass substrate is thin.
Now, this problem of noise generation is all the more accentuated as the optical shuttering devices must be integrated into the lightest possible devices, for example a viewing goggle set or a screen of a display device, requiring ever thinner glasses.
In other words, the weight reduction in devices that is sought by the manufacturers of these devices is limited by this noise generation coming from acoustic vibrations (or waves) that are propagated all the more easily as the thickness of the glass substrates is small. This problem is accentuated with the use of smectic liquid crystals because of significantly higher electrical fields than is the case with the nematic crystals.